Unfortuantely if you really want this you should go to something like addthis or sharethis, which isn't immediately helpful. If you could use "View as Webpage" as a token, you could embedd that URL into a social sharing button, but unfortunately you can't.
We use a token, {{lead.Social Buttons}}, that is managed by my Lead Creation process, such that when a new lead is created it assigns them the appropriate social media channels for their country. France has Viadeo, Germany has Xing, plus everyone has their own twitter feed, etc. But that isn't social sharing in the respect that they can share the content of that particular email, but rather connect with the corporate channels. Not the same.